Two hominid fossils discovered in Kenya are challenging a long-held view of human evolution.

The broken upper jaw-bone and intact skull from humanlike creatures, or hominids, are described in Nature.

Previously, the hominid Homo habilis was thought to have evolved into the more advanced Homo erectus, which evolved into us.

Now, habilis and erectus are thought to be sister species that overlapped in time.

The new fossil evidence reveals an overlap of about 500,000 years during which Homo habilis and Homo erectus must have co-existed in the Turkana basin area, the region of East Africa where the fossils were unearthed.

"Their co-existence makes it unlikely that Homo erectus evolved from Homo habilis," said co-author Professor Meave Leakey, palaeontologist and co-director of the Koobi Fora Research Project.

The jaw bone was attributed to Homo habilis because of its distinctive primitive dental characteristics, and was dated to around 1.44 million years ago.

It is the youngest specimen of this species ever found.

The skull, discovered by Frederick Manthi of the National Museums of Kenya, was assigned to the species Homo erectus despite being a similar size to that of a habilis skull. Most other erectus skulls found have been considerably larger.

But it displayed typical features of erectus such as a gentle ridge called a "keel" running over the top of the jaw joint. Analysis showed the skull to be about 1.55 million years old.

The new dates indicate that the two species must have lived side by side.

Sister species

If Homo erectus had evolved from habilis and stayed within the same location then both must have been in direct competition for the same resources.

Eventually, one would have out-competed the other.

There may have been a large size difference between the sexes

"The fact that they stayed separate as individual species for a long time suggests that they had their own distinct ecological niches, thus avoiding direct competition," Professor Leakey explained.

Professor Chris Stringer, head of human origins at London's Natural History Museum, said: "Both were apparently stone tool-makers, but one possibility is that the larger and perhaps more mobile erectus species was an active hunter, while habilis scavenged or caught small prey."

It is most likely that both species evolved from a common ancestor.

Other possibilities

But the linear, ancestor-descendent relationship between the two species cannot be ruled out altogether.

Fred Spoor, professor of developmental biology at University College London, and co-author of the paper, told the BBC News website: "It's always possible that Homo habilis lived, let's say, 2.5 million years ago and then in another part of Africa, away from the Turkana basin, an isolated population evolved into Homo erectus."

After a sufficient amount of time to allow both species to develop different adaptations and lifestyles, Homo erectus could have then found its way to the Turkana basin.

With separate "ecological niches", both species could co-exist without direct competition for resources.

"But that is a much more complex proposition," Professor Spoor explained, "the easiest way to interpret these fossils is that there was an ancestral species that gave rise to both of them somewhere between two and three million years ago."

Not so similar

The fossil record indicates that modern humans (Homo sapiens) evolved from Homo erectus.

However, to some researchers, the small size of the erectus skull suggests that species may not have been as similar to us as we once thought.

On average, modern humans display a low level of "sexual dimorphism", meaning that males and females do not differ physically as much as they do in other animals.

The scientists compared the small skull to a much larger erectus cranium found previously in Tanzania. If the size difference between the two is indicative of the larger one being from a male and the smaller being from a female, it suggests that erectus displayed a high level of sexual dimorphism - similar to that of modern gorillas.

Sexual dimorphism can relate to reproductive strategies and sexual selection.

If erectus was very sexually dimorphic it may have had multiple mates at a time. This differs from the more monogamous nature of modern humans, indicating that Homo erectus was not as human-like as once thought.

The researchers dismiss the idea that the small size of the skull could be a result of it belonging to a youngster.

"By studying how the skull bones are fused together we discovered it belonged to a fully grown young adult rather than a developing juvenile erectus," said Professor Spoor.